The present invention relates to an arrangement for applying a surface pressure to moving workpieces, such as wooden planks, rubber and plastic webs, etc.
More particularly, it relates to such an arrangement of the above type, which includes at least one rotatable pressing band which is pressed against a workpiece, a pressure plate which applies the working pressure to the operating travel of the pressing band, and a plurality of rolling bodies which are arranged in the region of the pressure plate at a distance from one another in frictional connection with the pressure plate and the associated operating travel of the pressing band, which extend over the whole width of the pressing band, and are transported back in a closed path from the outlet end of the pressure plate to the inlet end of the pressure plate.
A double-band press of this type is disclosed, for example, in the German document DE-OS No. 31 17 778. In this machine the rolling bodies arranged between the pressure plates and the pressing bands are formed as rolling rods. These rolling rods extend over the whole width of the machine which can amount to two or three meters, and have a diameter from fourteen to sixteen millimeters. Such an unfavorable length/diameter ratio leads to a high torsion loading of the individual rolling rods or to a slippage of the rolling rods between the pressing band and the pressure plate which are generally made of steel sheets, and therefore to a premature wear. The rolling rods have diameter differences which can be caused by manufacturing tolerances, manufacturing inaccuracies, different heat expansion, etc. As a result of these diameter differences of the rolling rods, different points of the rolling rods have different angular speeds during rolling through a predetermined path in the pressing region. Therefore the rolling rods, in dependence on their diameter tolerances, are subjected to different torsion stresses distributed over their length.
For avoiding breakage of such rolling rods under the action of these torsion stresses, it is known to operate with small diameter tolerances, which should advantageously be for example lower than 10 micrometers. The manufacture of rolling rods with a length of over two meters and a diameter from fourteen to sixteen millimeters, with diameter tolerances under 10 micrometers, is very difficult and expensive. The high torsion of the rolling rods cannot be avoided even with relatively small manufacturing tolerances.
It should be mentioned that in addition to the diameter tolerances in the individual rolling rods, there are also further interfering or disturbing size deviations from one rolling rod to another. Together they lead to many difficulties.